For general illumination, it is desirable for lighting fixtures to be dimmable. Many control circuits for lighting use phase cut dimming. In phase cut dimming, a portion of the AC waveform, for example, the leading or trailing edge, is blanked (“cut”) to reduce the RMS voltage provided to a lighting device. When used with incandescent lamps, this reduction in RMS voltage results in a corresponding reduction in average current and, therefore, a reduction in power consumption and light output. As the RMS voltage decreases, the light output from the incandescent lamp decreases.
Recently, solid state lighting systems have been developed that provide light for general illumination. These solid state lighting systems utilize light emitting diodes or other solid state light sources that are coupled to a power supply that receives the AC line voltage and converts that voltage to a voltage and/or current suitable for driving the solid state light emitters. Typical power supplies for light emitting diode light sources include linear current regulated supplies and/or pulse width modulated current and/or voltage regulated supplies.
Dimming circuits for solid state lighting components have been developed. However, it has been found that dimming a solid state lighting apparatus can cause the chromaticity of the light output by the apparatus to change.
Solid state devices, such as light emitting diode (LED) chips may be tested and grouped and/or binned according to their respective output and/or performance characteristics prior to being mounted in an LED package. The grouping may be performed using, for example, chromaticity values, such as the x, y values used in the CIE 1931 chromaticity diagram that was created by the International Commission on Illumination in 1931. In this manner, each light emitting device may be characterized by x, y coordinates. Emitters having similar x, y values may be grouped or binned to be used together, i.e., to be mounted together in a single LED package. LEDs and/or LED packages may be combined in a solid state lighting apparatus, and the combined light emitted by the LEDs in the apparatus defines the chromaticity of the light emitted by the apparatus. Typically, the chromaticity of the apparatus is specified as a design parameter, and it is undesirable for the chromaticity to change significantly during operation of the apparatus.